Teachers in K-12 settings increasingly demand instructional materials beyond textbooks to follow the upward momentum of personalized instruction. Especially during the outbreak of COVID-19, K-12 teachers are forced to quickly adapt to online teaching and thus have more difficulties of delivering personalized instruction in a relatively resource-restraint situation. Open educational resources (OER), allowing teachers to retain, reuse, revise, remix, and redistribute high-quality educational resources at no costs, can be a viable option for teachers. However, the acceptance of OER in K-12 education still remains low. Effective strategies are needed to reinforce teacher intention to adopt OER. This research showcased a two-phase sequential explanatory mixed method inquiry to investigate whether engaging K-12 teachers in open educational practices (OEP)—such as renewable assignments—increased their acceptance of OER. The quantitative phase, referring to the technology acceptance model (TAM), examined the change in factors influencing teachers’ acceptance of OER. The qualitative phase was followed up to provide supplementary perspectives of the quantitative findings. By integrating complementary findings, this research found that OEP increased teachers’ perceived ease of and self-efficacy towards using OER. Although teachers’ intention of implementing OER is not significantly improved, qualitative findings offered additional insights into the benefits of OEP in promoting OER usage and the recommended directions for future effort. Practical implications on improving teachers’ acceptance of OER in K-12 curriculum are discussed at the end.